Expert 5K+
Posts: 9908
Location: Lower Mainland BC | .
I am not the expert you need but I know a little bit.
Let me say this: The factory 1955 Chrysler 300 with two 4bbls on a 331 cu engine that made 300 hp used mostly Powerflite 2 speed automatic transmissions.
REFERENCE: http://oldcarbrochures.org/United%20States/Chrysler_and_Imperial/19...
So, in general, a Powerflite would probably be okay on your warmed over 291 Desoto. That said, the 1955 300's Powerflite was beefier than most Powerflites (more clutch discs, etc) and probably used a water cooled torque converter as shown in these images (one is from a Desoto to it might be similar to your set up - if it is, that is a good indication that your Powerflite might already be up to the task) (Scroll down to see the images):
See if you can see the number that is stamped on a machined spot near the filler tube on the right side of the transmission and then report back (this - from bbdakota):
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